Early evolutions in time measurement included the use of water clocks, pendulums, and quartz. However, in 1967 a new standard of measurement was devised. The cesium atom's natural frequency was formally recognized as the new international unit of time: the second was defined as exactly 9,192,631,770 oscillations or cycles of the cesium atom's resonant frequency. The second quickly became the physical quantity most accurately measured by scientists. Our best primary cesium standards now keep time to about one-millionth of a second per year.

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